The present invention relates to a probe head of the switching type, such as used, for example, in coordinate-measuring machines (CMM) or for the measuring of workpieces on machine tools. Much a probe head is described in U.S. Pat. No. 4,177,568.
Probe heads of the switching type have a positioning support which establishes with high precision, the zero position of the probe-pin carrier. The probe-pin carrier is the movable part of the probe head and is located in the zero position prior to a work-contacting procedure; and the probe-pin carrier returns to the zero position after each contacting procedure. This positioning support is, as a rule, developed as a statically determined threepoint support and illustratively consists of three pairs of balls lying in a plane, for unambiguous seating engagement with radially protruding cylinders; alternatively, the positioning support may consist of three V-shaped notches into which three balls engage, and there are other technically equivalent embodiments. All of these supports, however, have the disadvantage that, due to customary spring-loading of the movable part in the direction of zero-position seating, the course of probe deflection is very asymmetrical for lateral work-contacting displacements; the asymmetry is various depending on the direction of work-contacting displacement, since the probe-pin carrier can tilt one time over one support point and the next time over the line connecting two support points, in which case the tilt axis is closer to the axis of the probe pin. As a consequence, work-contacting forces are unequal, and measurement uncertainties result, since different workcontacting forces can also give rise to different bends of the probe pin, before developing the electrical workpiece "contact pulse" which is relied upon to characterize the point of probe-pin contact with the surface of the workpiece.
This problem is already known and there has been no lack of attempts to solve the problem by providing statically overdetermined supports for the movable part of the probe head, such that in its zero position the probe-pin carrier lies either on a very large number of points or on an annular support part.
For example, Federal Republic of Germany OS 1,937,154 describes a stylus-operated copying device having a movable part which is dish-shaped in the region of its support and having an outer periphery which, in its position of rest, engages the plane of an annular resting surface on a fixed part of the copying device. A two-layer membrane centers the movable part in the annular plane and urges the same axially against the annular supporting surface. For this purpose, the two individual membranes are spread somewhat with respect to each other in the position of rest. This arrangement is not satisfactory for measurement purposes, since centering stability is relatively poor. Furthermore, the two individual membranes are edge-connected, so that only relatively slight deflection movements can be tolerated if the membrane is not to be overstressed.
U.S. Pat. No. 3,566,479 describes a probe head of the measuring type, wherein the movable part is centered by a single-layer membrane (FIG. 14). This single-layer membrane is fastened to the movable part and in its position of rest either lies against an annular collar on the stationary part, or is urged against it. But this type of support also permits only slight deflection. Furthermore, with this construction, the angular position of the movable part in its position of rest is determined with only very slight precision, due to the small diameter of the annular collar and the radial distance between the movable part and the annular collar.
European Patent No. A2 0,303,831 describes a probe head of the switching type having a probe-pin carrier that is centered by five disks lying one above the other, the disks being in each case connected in pairs to each other at their ends and permitting probe-pin carrier displacement on defined axes which are parallel to the coordinate directions. However, the probe-pin carrier lies on three or four support points in the direction of the axis of the probe pin. This known probe head, however, is also unsatisfactory, for its lack of precision in restoring the probe-pin carrier to its at-rest position, i.e., centering precision and uniformity of work-contacting force are both unsatisfactory. Furthermore, the involved probe head requires relatively large space due to the rectangular arrangement of the tilting axes.